1. Field of the Invention
This invention relates generally to a voice coil type actuator, and more particularly to a voice coil type actuator wherein a movable body having a movable coil supported thereon, a guide shaft having the movable body supported for sliding movement thereon and an outside yoke and an inside yoke for defining a closed magnetic path along which a magnetic flux produced from a magnet passes are disposed in a coaxial relationship with each other and a movable member located on the outer side of the outside yoke is connected to the movable body such that the movable body is moved by a moving force exerted by the movable coil.
2. Description of the Prior Art
An exemplary drive for moving a movable member along a linear passage such as, for example, for moving a movable lens in a lens system of a camera or for moving a recording and reproducing head of a disk type recording and reproducing apparatus is a voice coil type actuator or drive.
An actuator of the type mentioned is normally constructed such that a movable body which has a movable coil supported thereon is in turn supported for sliding movement on a guide shaft or the like and a magnetic field forming element including a magnet and a yoke held in contact with each other is disposed such that an annular space in which the movable coil is located is formed therein. A closed magnetic path in which magnetic fluxes pass the movable coil in a direction perpendicular to an axial direction of the movable coil is formed, and further, the movable body and a movable member are connected to each other such that the movable member is moved by a moving force exerted by the movable coil.
An example of a conventional voice coil actuator having such construction is disclosed in Japanese Patent Laid-Open Application NO. 1-196011 and is shown in FIG. 7 of the present application.
Referring to FIG. 7, the actuator, which is generally denoted at a, includes a housing b secured to a lens barrel c. A pair of bearings d are secured to the opposite forward and rearward end portions of the housing b, and a sliding shaft e is supported by the bearings d at the opposite end portions thereof for sliding movement in an axial direction thereof. A yoke body f is secured to the housing b in a coaxial relationship with the sliding shaft e. The yoke body f has an outside yoke portion g of a cylindrical shape and a front wall portion h which closes an end face of the cylindrical outside yoke portion g. A magnet i is mounted in a coaxial relationship with the outside yoke portion g. The magnet i is substantially ring shaped and is magnetized so that the opposite polarities are in the axial direction. An inside yoke j also having a substantially ring-shaped profile is mounted on a rear face of the magnet i, and a closed magnetic path along which magnetic fluxes produced by the magnet i pass is formed by the yoke body f, the magnet i and the inside yoke j. The path of the magnetic fluxes is indicated by a broken line arrow in FIG. 7.
A movable body k of the conventional actuator has a main portion 1 having a substantially disk-like profile, a hub portion m extending forwardly from a central portion of the main portion 1, and a coil bobbin n of a cylindrical shape which extends forwardly from an outer periphery of the main portion 1. The hub portion m of the movable body k is fitted around and secured to the sliding shaft e, and a movable coil o is wound on an outer periphery of the coil bobbin n. The movable coil o is positioned in an annular space defined by the inside yoke i and the outside yoke portion g of the yoke body f, and accordingly, magnetic fluxes from the magnet i flow through the movable coil in a direction perpendicular to an axial direction of the movable coil o. A lens holding member p has a lens holding portion q of a substantially cylindrical shape and a connecting portion s extending from a rear end portion of an outer periphery of the lens holding portion q. The lens holding member p is secured at the connecting portion s thereof to a face of the main portion 1 of the movable body k remote from the inside yoke j, and a movable lens r is held by the lens holding portion q of the lens holding member p.
When a driving electric current is supplied to the movable coil o, a moving force is exerted on the movable coil o in a direction corresponding to the direction of the driving electric current, and the movable body k and the sliding shaft e and the lens holding member p are moved integrally by the moving force.
The voice coil type actuator a described above if subject to the following problems.
In particular, since interconnection between the movable body k on which the movable coil o is supported and the lens holding member p and the movable lens r which serve as a movable member is at a position spaced from the movable coil o in the axial direction, there is a problem that the dimension L required for establishing such an interconnection increases the size of the actuator a in the axial direction.
Further, since the conventional actuator a employs parts for exclusive use each having a single function for the two inner and outer yokes, the magnet, the guide shaft and so forth which are essential elements to form a magnetic field having a closed magnetic path, there is a problem that it is difficult to reduce the number of parts without deteriorating any of their functions.
Furthermore, since the space defined by the outside yoke portion g of the yoke body f and the inside yoke j at least has a length equal to a range of movement of the movable coil o, the magnetic flux density in the space increases toward the magnet i and is not uniform, and accordingly, there is a problem that, as the range of movement of the movable coil o, or in other words, the range of movement of the movable member, increases, movement of the movable coil o likely becomes unstable as such.
Besides, while the front face portion h of the yoke body f is formed of a comparatively small thickness to make the size of the actuator a in the axial direction as small as possible, if the thickness of the front face portion h is too small, then saturation of magnetic fluxes takes place here, and after all, there is a problem that it causes a reduction in the magnetic flux density in the space described above.